U.S. Pat. No. 3,399,085 discloses a process whereby the surface of nitriding steels can be readily nitrided to produce a well-hardened case without the formation of the undesirable brittle outer skin known as "white layer".
In the practice of the patented process, the nitriding time should not depend on the surface area being nitrided. Experience has shown that no problem is encountered in choosing the nitriding time to produce a satisfactory case with a predictable hardness profile as long as a relatively large amount of the specified dry NH.sub.3 --H.sub.2 gas mixture is allowed to flow over a comparatively small work load, e.g. 165 cu. ft. of gas per minute per 100 sq. ft. of steel surface area being nitrided in a non-porous-alundum reactor. There is, however, a serious size limitation on the area of steel that can be nitrided if this flow rate is not maintained particularly in an uncoated Inconel reactor. That is to say, at much lower flow rates the nitriding time needed to produce a given hardness profile can no longer be estimated.
This failure to effect suitable and reproducible nitriding in large areas of steel has been attributed to a drop in concentration of NH.sub.3 in the gas mixture which is caused primarily by its decomposition to nitrogen and hydrogen. The problem was, therefore, in part overcome by working at temperatures near the higher end of the permissive range, employing higher concentrations of NH.sub.3 and larger flow rates of the nitriding gas mixture. Such practices, however, add considerably to the cost of the operation and do not eliminate the time selection difficulty.
U.S. Pat. No. 3,684,590 discloses a practice wherein the above problems are overcome. The patented practice is based in part upon the discovery that the above-mentioned difficulties are usually not the result of a reduction of NH.sub.3 concentration as had been believed, but rather are caused by the generation of impurity gases such as hydrogen cyanide, HCN, in side reactions during nitriding, which inhibit the nitriding reaction. These nitriding inhibitors, or poisons, contaminate the nitriding gas somewhat in proportion to the surface area of the steel being nitrided. Amounts of HCN as little as ten parts per million, can cause excessive and erratic retardation of the nitriding reaction. Pursuant to the patented process therefore, the NH.sub.3 --H.sub.2 nitriding atmosphere is recirculated so that nitriding inhibitors can be removed and so that the moisture content can be regulated as desired to minimize formation of nitriding inhibitors. Specifically, the nitriding atmosphere is circulated from the nitriding furnace to a gas-to-gas heat exchanger where the temperature thereof is lower to a preselected level. Thereafter, the atmosphere is conveyed through a thermostated scrubber containing an aqueous alkaline solution which removes HCN and other nitriding inhibitors. The atmosphere is precooled so that the scrubber temperature can be maintained at a predetermined level, i.e. thermostated, to thereby control the water partial pressure in the atmosphere within the desired range of 7 to 20 torrs depending on the concentration of the aqueous caustic solution. The scrubbed atmosphere is then returned to the nitriding furnace via the heat exchanger.